Modern day computer systems have many modules and printed circuit boards which involve specialized power requirements to be derived from the main AC source.
Because different voltages and currents are involved in different parts of a computer system, it is necessary that the original AC power input be distributed to various areas of AC application without destruction of the internal components due to power line surges or inrush currents.
The earlier forms of power distribution and control circuitry used in computer systems were very bulky and involved complex status and control circuitry which increased the size of the power distribution unit (PDU) and required additional cooling fans. Additional wiring and power supplies increased the size and packaging of these units. These earlier types of power distribution units required digital status and control circuitry located within them, requiring extra cabling, connector terminations, electronic and electromechanical components. This costly complex circuitry resulted in lower level of "meantime between failures" (MTBF).
Different voltages and currents are often required in different areas of a computer system. It is necessary to distribute the primary AC source power to various AC loads within the computer system without damage to internal components or disruption of computer operation which could occur as the result of power line surges or inrush currents.
Earlier designs of computer system power distribution units (PDUs) were physically large and bulky. This considerable size resulted in packaging difficulties and need for increased overall cabinet dimensions. Traditional PDU designs include digital status monitoring and control circuitry within the PDU. Their circuitry required additional DC power supplies, electromechanical contactors, wiring, connectors and cooling fans to accommodate the generated heat.
Typical prior art power distribution systems involving sequential application of power to various loads include such systems as described in U.S. Pat. No. 4,769,555, entitled "Multi-Time Delay Power Controller Apparatus with Time Delay Turn-on and Turn-Off" and also U.S. Pat. No. 4,663,539, entitled "Local Power Switching Control Subsystem". However, these systems cannot provide the versatility, flexibility and programmability inherent in the presently described system.
The inclusion of monitoring and control circuitry within the PDU creates six basic liabilities: 1) unnecessary electrical paths; 2) extra harnesses and cables; 3) increased troubleshooting time; 4) increased parts count; 5) lower reliability; 6) increased overall equipment costs.
The typical computer "disc drive" is controlled by circuitry mounted close to, or on the drive assembly. This circuitry, in turn, is controlled by an "external" disc controller outside both the drive assembly and the P.D.U.
Depending upon the specific design, the disc controller lines carry signals and/or DC levels to and from the PDU. The disc controller monitors the signals and/or levels and transfers the appropriate response to the drive circuitry. The circuitry involved is inappropriately located in the PDU, thus creating the liabilities described above.
A PDU (Power Distribution Unit) has four basic operational requirements: 1) select discrete paths over which primary power can be distributed to AC loads; 2) capable of remote ON/OFF control; 3) capable of local ON/OFF control; 4) capable of emergency power-down function. These basic characteristics apply to all PDUs.
The present invention provides all the basically valuable characteristics while eliminating extra circuitry formally located in the PDU, while also eliminating damaging in-rush currents.
Another frequent problem of these earlier types of power distribution units where AC loads were composed of AC- to -DC power supplies, involve the situation where excessively high input currents could be present at the power distribution unit input at the very instant that the alternating current power was applied. For example, if the power distribution unit (PDU) was connected to eighteen auxiliary power supplies each having individual inrush currents of 24 peak amperes, it would theoretically be possible for a resultant current surge of 432 peak amperes to occur and cause damage to circuitry components as a result.
The problem of excessive and damaging inrush currents are obstacles which occur in power supply distribution systems. Excessive electrical power line inrush currents cause temporary or permanent damage to electrical circuitry and/or cause equipment malfunctions or safety hazards.
An explanation of INRUSH CURRENT requires a perspective as follows:
1. "Inrush" current is not the same as surge current. The terms Inrush Current and Surge Current cannot be used interchangeably. PA1 2. FIG. 5 shows a single resistor-capacitor load configuration connected to a single phase source. FIGS. 10 and 11 represent three different loads (power supply inputs A, B & C) connected in parallel to the same source, as shown in Block Diagram FIG. 9. FIG. 12 represents loads A, B and C connected to the power source as shown in Block Diagram, FIG. 13.
Inrush refers to values of load current amplitudes existing during the first few cycles of applied power to an electrical AC load. The most significant value of current exists during the first half cycle of applied power. "Inrush" is the result of the permanent design characteristic of the load. PA2 "Surge Current" refers to a sudden increase in load current amplitude above a normal value of continuous constant amplitude. An example of surge current would be an increase to 50 amperes from a normal operating current of 5 amperes, as a result of a sudden load malfunction, or an external line stimulus such as a lightning strikes on a power line. Surge current is not a permanent characteristic of a properly designed load. Surge current, in this context, correlates to circuit damage or operational interruption. Any load can be designed to accommodate certain surges, rendering them non-destructive. PA2 Both inrush and surge effects are dependent upon amplitude and duration of the condition. Lightning strikes (or any excessive line voltage increases) create the possibility of circuit voltage breakdown.
FIG. 8 shows the capacitor current curve and the resultant line current curve after power has been applied.
In FIGS. 6, 10, 11, and 12, the positive half of each cycle is shown.
Prior to closing the switch in FIG. 5, the capacitor has been discharged by the load resistor. Upon switch closure at time T1, FIG. 6 (input voltage peak) the capacitor appears as a short (zero ohms resistance) to the voltage applied to it. The line current rises to a maximum; limited by the amplitude of applied voltage, diode resistance, and effective series resistance of the capacitor. The load resistor current is insignificant because of the relatively higher resistance of the load, compared to the capacitor. In FIG. 6, between time T1 and the beginning of Cycle 2, the source voltage has decreased to zero, gone through the negative half cycle, and returned to zero. The diode cuts off the negative portion of the cycle, to cause zero line current during the last half of Cycle 1.
From time T1 to the beginning of Cycle 2, (FIG. 6) the capacitor C1 has partially discharged. Capacitor discharge is through the relatively high resistance of the load resistor. Full discharge cannot occur before the beginning of Cycle 2. At the start of Cycle 2, the capacitor has a partial charge remaining in it. The partial charge on the capacitor permits it to attain a full charge during the positive half of Cycle 2 with less current flow. The peak capacitor current during Cycle 2 is lower than Cycle 1 because of the partial charge present at the start of Cycle 2.
In Cycle 3, FIG. 6, the exact same capacitor charge and discharge sequence occurs, with line current fluctuations leveling off at some value. The charge and discharge fluctuations in the capacitor, beginning at time T3, create an AC current known as "ripple".
The preceding explanation was related to a standard half wave power supply configuration. When a full wave configuration is used, both halves of the input AC power cycles will be rectified. The resultant line current would appear similar to FIG. 7.
FIG. 8 shows line current inrush with a "full wave" power supply configuration. Each half cycle decreases in amplitude by a lesser amount until the current level is stabilized. This is because the same capacitor is being charged during both halves of the rectified cycle.
The prior half wave explanation in FIG. 7 assumed switch closure at the instant the AC input voltage first half cycle was at its peak. That is the time when maximum inrush current occurs. If the switch has been closed at any other time during that first half cycle, an inrush current of lower magnitude would have been observed.
The following discussion describes the additive effects of three inrush currents created by three power supplies connected in parallel to the same source as shown in FIG. 9 where inrush currents occur simultaneously to each of the power supplies A, B, C.
When a group of power supplies is connected in parallel and controlled by one switch, power is applied to all supplies at the same instant. Slight differences in component characteristics may cause each of the supplies to charge its capacitor at a different rate. FIG. 10 shows inrush currents to power supplies A, B and C. Each power supply, operated "alone", would have a peak current of 24 amps. Each line current is displaced from the others by some time period dependent upon component characteristics--even though input power has been applied to all supplies at the same instant.
In FIG. 10 power supply A and B line currents are shown displaced. At the instant power supply A current has peaked at 24 amps, power supply B current has risen to 10 amps. The vertical addition of the two currents on the graph, results in a 34 ampere line current at that instant and shown by the dashed line. At this instant, the resultant line current amplitude is greater than the current in either power supply A or B.
Referring to FIG. 11, three power supplies A, B and C are connected in parallel. All components characteristics are identical. Power has been applied to all supplies at the same instant. Capacitor charge times are identical. Each individual supply has an inrush current of 24 amperes. All three current peak times are coincident. Under these conditions, the vertical addition of the currents is seen to be 72 amperes.
If, for example, 20 power supplies were connected in parallel, with 8 ampere running currents and 24 ampere inrush currents, the running current would be 160 amperes. With uncontrolled inrush currents, the possibility of a 480 ampere inrush is present. The danger to circuitry and components is clearly evident in this situation.
A solution to the problem is to "sequence" power to the individual power supplies. Referring to FIG. 12, when power is applied to power supply A for three full cycles, the first half cycle inrush will have dissipated and the line current stabilized at 8 amperes. At this time, power is applied to power supply B for three full cycles. When power supply B has stabilized, power can be applied to power supply C without regard to further timing.
In the group of 20 supplies previously mentioned, the greatest inrush current to be expected would be sum of the first nineteen (each stabilized after 3 cycles) 8 amp running currents (152 amps) plus the inrush of the last supply to turn on, (24 amps). Under controlled conditions, the maximum inrush would then be 176 amps.
The present high current power distribution control system eliminates the earlier need for on board power supplies, excessive hardware wiring, and digital electronic circuitry--thus permitting the physical size of the power distribution unit to be significantly reduced. Further, since fewer parts and components are involved reliability is consequently improved and the operational life of the power distribution unit considerably lengthened.
The simplified circuitry permitted by the presently described power distribution unit system permits an almost universal usage in any number or types of electronic systems without the necessity to change the basic power distribution unit circuitry. Further, the need for repairs to the power distribution unit is considerably reduced and involves a reduced inventory of extra spare parts. Additionally, the minimized power distributor unit and system lends itself to efficient cooling by attachment to the cabinet frame as indicated in FIGS. 4A and 4B. Thus the metallic frame, A.sub.f, is so based as to provide solid contact, as seen in FIG. 4B, to each control unit, CU1, CU2, CU3, and to each solid state relay, K.sub.1, K.sub.2, K.sub.3, and to the circuit breaker units CB.sub.1, CB.sub.2, CB.sub.3. The metallic frame also functions as a heat sink for the control units (CU), the solid state relay units (K), and the circuit breaker units (CB).
It is an object of the present system to reduce the damaging inrush current which can occur by dividing the power sequencing function into separate turn-on periods. By sequencing the application of power to at least three separate groups of power supplies, it is possible to reduce the instantaneous result if, for example, eighteen power supplies were powered up simultaneously.
It is a further object to provide a compact power distribution system which can be installed onto a cabinet frame which holds digital modules where the frame can act as a heat sink.
Another object of the system is to provide a programmable arrangement for individually controlling each one of a multiple set of power channels.